1. Field of the Invention
The invention relates to sonar particularly with respect to linear hydrophone arrays for use in narrow underwater vehicles. The term "hydrophone array" is construed to include hydrophone/projector arrays.
2. Description of the Prior Art
Underwater vehicles generally utilize a forward-looking sonar for obstacle avoidance, mine detection, etc., a linear hydrophone array typically being mounted in the nose of the vehicle. Some of these vehicles tend to be small in diameter. A current evolutionary trend is toward Autonomous Underwater Vehicles (AUV) which can be launched from a submarine torpedo tube. Such an AUV is thus limited to a 21 inch diameter.
The imaging resolution achievable by the sonar is limited by the width of the linear array at a given sonar frequency. The wider the array, the better the resolution. Resolution can also be enhanced by utilizing higher sonar frequencies, but the higher the frequency, the lower the range capability of the sonar. Wide arrays can accommodate moderate resolution at a lower frequency, a lower frequency permitting greater range capability. Small vehicles, however, such as AUVs, are too small to accommodate wide arrays while stowed in their launch tubes or carrying fixtures.
For most underwater vehicles, design constraints limit the diameter of the vehicle body and, thus, the size of sonar hydrophone arrays contained within the body. For vehicles launched from a torpedo tube, the constraint is exacerbated. Because of these constraints, such vehicles have limited width hydrophone arrays resulting in poor resolution sonar data. Alternatively, the sonar design is limited to the use of relatively high frequencies which reduces sonar range capability.
The evolution in AUVs is toward reducing vehicle size to accommodate various methods for deployment; e.g., the 21 inch torpedo tube launchable vehicle class. However, reduction in overall vehicle size increases the need for greater vehicle autonomy resulting in a requirement for improved on-board sensors. As discussed above, however, improved resolution sonar sensors for a given sonar frequency tends to require a larger vehicle. The larger vehicle size requirement impedes the evolutionary trend toward smaller vehicles.